This invention relates to ink jet head arrangements and, more particularly, to a new and improved ink jet head arrangement providing a compact and highly effective array of ink jets in a convenient and efficient manner.
In conventional ink jet heads ink which is held for a period of time adjacent to the ink jet orifice while the jet is not operating tends to absorb air from the atmosphere. When the ink jet is subsequently actuated, decompression of the ink adjacent to the jet orifice when negative pressure is applied during the operating cycle of the ink jet may cause bubbles to form in the pressure chamber adjacent to the orifice. Such bubbles must be removed from the ink to avoid interference with the operation of the ink jet.
In ink jet systems using thermoplastic, or hot melt, inks, cooling and solidification of the hot melt ink in the region adjacent to the jet orifice when operation of the systems is terminated causes the ink to contract, drawing air inwardly through the orifice into the pressure chamber. As a result, the next time the ink is melted to prepare the system for use, the pressure chamber contains air bubbles which, as pointed out above, will interfere with operation unless they are removed. Furthermore, where hot melt inks containing pigment are used, the pigment can settle out of the ink and agglomerate during quiescent periods of time when the ink is kept in the molten condition but the ink jet is not being used.
To reinforce the positive pressure pulse developed by a piezoelectric crystal to eject an ink drop through the orifice of an ink jet, it has been proposed to provide a large-capacity chamber communicating with the end of the pressure chamber adjacent to the ink supply to provide a low acoustic impedance to pressure pulses from the chamber so that a negative pressure pulse applied to the pressure chamber by the piezoelectric crystal will be reflected by the low acoustic impedance chamber back through the pressure chamber as a positive pulse which is then reinforced by the piezoelectric transducer as it moves toward the ink jet orifice to eject a drop of ink. Such large-volume, low acoustic impedance chambers, however, require a very large structure for the ink jet head, preventing a compact array of closely spaced ink jets. Furthermore, if two ink jet orifices are connected to the same ink supply line, operation of one ink jet tends to influence the operation of the other ink jet connected to the same supply line, producing a cross-talk condition. More over, the spacing of ink jet orifices in an ink jet array has generally been limited by the minimum width of the pressure chambers communicating with the orifices which is usually about one millimeter.